BEIRUT (AP) -- Hundreds of Syrian civilians streamed out on foot from the eastern part of the city of Aleppo on Friday in the wake of the relentless campaign by government troops and their allies to drive rebels from their rapidly crumbling enclave.

The U.N. human rights office said it was deeply concerned about reports that hundreds of men have gone missing after crossing from eastern Aleppo into government-controlled areas of the city.

Spokesman Rupert Colville said that family members have reported losing contact with the fighting-age men, who are between 30 and 50 years old, after they fled opposition-held areas of Aleppo around a week or 10 days ago. It was not clear whether they were fighters or civilians.

Colville also said the U.N. High Commissioner for Human Rights is also concerned by reports that some civilians trying to flee are being blocked by armed opposition groups and in some cases being fired upon.

"Civilians are being used as pawns and prevented from leaving," he said at a briefing in Geneva. He estimated there may currently be around 100,000 civilians in areas under the control of armed opposition groups in eastern Aleppo. They include about 500 medical cases involving people in need of urgent evacuation.

Footage broadcast Syrian state TV showed families emerging from Aleppo's ravaged eastern districts, as civilians streamed out of the enclave that had been held by rebels since 2012.

The government-owned al-Ikhbariya TV showed civilians on foot and at least one bus snaking through the Ballour crossing, and said they were coming from the Saleheen, Fardous and Sheikh Saeed neighborhoods in the southern part of eastern Aleppo.

The exodus came a day after Russia announced that the Syrian army was suspending combat operations to allow for the evacuation of civilians from besieged rebel-held districts, but residents and medics inside eastern Aleppo said there was no letup in the bombardment.

"Bombing is truly round the clock," said Ziad Mohammed, a lawyer and father of three, still living in eastern Aleppo's al-Mashhad neighborhood. "There are no hospitals, the remnants of the dead fill the streets and the wounded have to fend for themselves."

Mohammad, an outspoken government opponent, said he and many of those remaining were bracing themselves for certain death. "If staying here means dying here, then standing by my principles will have been enough," he said.

Earlier this week, efforts to evacuate hundreds of wounded people faltered, despite pleas from medical officials. A hospital administrator in eastern Aleppo said medics have submitted lists of patients who need to be evacuated from hospitals.

"There hasn't been a response yet, and the shelling continues," he said, speaking on condition of anonymity for fear of retaliation.

Rebel defenses have collapsed in the face of a wide-ranging government offensive preceded by an intensive aerial campaign. More than three-quarters of the rebel sector has now fallen, including the symbolically important ancient Aleppo district.

Tens of thousands of residents have fled to western Aleppo, and those remaining are now cornered in the southern part of the formerly rebel-held east.

An opposition official disputed government claims that the army now holds 90 percent of eastern Aleppo, saying that more than a dozen neighborhoods remain in rebel hands.

"More than thirty percent of east Aleppo is controlled by us," said Osama Abu Zayd, an adviser to the loose rebel umbrella group known as the Free Syrian Army. Speaking from Turkey, where he is based, he said the Syrian army and Russians were still targeting rebel positions on the front lines.

In Russia, Lt. Gen. Sergei Rudskoi of the military's General Staff said on Friday that 10,500 civilians - including 4,015 children - have left Aleppo's eastern neighborhoods in the last 24 hours. The number could not be independently confirmed.

Russian Foreign Minister Sergey Lavrov, speaking in Germany after talks with U.S. Secretary of State John Kerry, said military experts and diplomats would meet Saturday in Geneva to work out details of the rebels' exit from Aleppo's eastern neighborhoods, along with civilians who are willing to leave the city.

In central Syria, meanwhile, Islamic State militants advanced on government positions in the countryside around the historic city of Palmyra, according to the Britain-based Syrian Observatory for Human Rights, which tracks the Syrian conflict. It said at least 49 soldiers were killed in 24 hours of fighting in the desert.

The IS group's Aamaq News Agency distributed video showing what it says were Syrian soldiers fleeing their positions in the badlands west of Palmyra.

The Syrian government, backed by Russian forces, recaptured Palmyra from IS in March, to great fanfare. The militants had destroyed numerous monuments dating back to Roman times during their hold of the city.

The two militaries have since diverted their attention to fighting local opposition forces around Damascus and Aleppo.

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Associated Press writer Dominique Soguel in Istanbul, Philip Issa in Beirut, and Natalya Vasilyeva and Vladimir Isachenkov in Moscow contributed to this report.